Interest in electrical conductivity has increased in recent years in order to provide electrically conductive layers on various articles or elements. Work has intensified with emphasis on polymers having extended configuration in the backbone chain. One conjugated polymer system currently under considerable investigation include unsubstituted or substituted polyanilines, as noted in WO 92/22911 (published Dec. 23, 1992).
Problems associated with the formation and discharge of electrostatic charge during the manufacture or use of imaging elements have been recognized for many years by workers in the imaging arts. The accumulation of charge on imaging elements leads to the attraction of dust which can cause defects. Additionally, the discharge of accumulated charge during or after the application of imaging layers, such as photographic emulsion layers, can produce irregular fog patterns or "static marks" in the layer, as well as repellency spots and other undesirable defects. The severity of static problems has increased due to new emulsions of increased sensitivity, dramatic increase in coating speeds and post-drying efficiencies. The instances in which static charge can be accumulated and discharged during manufacturing, packaging, storage, and use are innumerable, and a solution to this problem has been actively sought in the industry for many years.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically conductive layers in an imaging element. Such antistatic layers can be applied to either or both sides of the element, over or under imaging layers, and on the same or different side of the support as the imaging layers. For some applications, an antistatic agent can be incorporated into the imaging layer, or into the support, or into both.
The requirements of antistatic layers in silver halide photographic films is especially demanding because of the stringent optical requirements. Other types of imaging elements such as photographic papers and thermal imaging elements also frequently require the use of an antistatic layer, but the requirements are less stringent.
Electrically conductive layers are also commonly used in imaging elements for purposes other than providing static protection. For example, in electrostatographic imaging, it is well known to use imaging elements comprising a support, an electrically conductive layer that serves as an electrode, and a photoconductive layer that serves as the image-forming layer.
A wide variety of materials can be used as antistatic agents in various locations in the element. Most of the traditional antistatic layers comprise ionic conductors. Thus, charge is transferred in ionic conductors by the bulk diffusion of charged species through an electrolyte. The prior art describes numerous simple inorganic salts, alkali metal salts or surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, colloidal metal sols, metal halides mixed in an organic polymer matrix or a matrix of a colloidal inorganic colloid such as silica. Conductivity of most antistatic agents is generally strongly dependent upon temperature and relative humidity of the environment as well as the moisture in the antistatic layer. Simple inorganic salts are usually leached out of antistatic layers during processing, thereby lessening their effectiveness.
Antistatic layers employing electronic conductors have also been described in the art. Because the conductivity depends predominantly upon electronic mobilities rather than ionic mobilities, the observed electronic conductivity is independent of relative humidity and other environmental conditions. Such antistatic layers can contain conjugated polymers, metal oxides, doped metal oxides, conductive carbon particles or semi-conductive inorganic particles. While such materials are less affected by the environment, a lengthy milling process is often required to reduce the particle size range of oxides to a level that will provide a transparent antistatic coating needed in most imaging elements. Additionally, the resulting coatings are abrasive to finishing equipment.
It is also known to prepare antistatic layers from electronically conductive polymers. Various polymer dispersions have been prepared as "loaded" or sorbed in latex polymers (see U.S. Pat. No. 4,237,194 of Upson et al). The resulting semiconductor-loaded polymer dispersion can then be coated on a variety of photographic supports. However, such layers have a slightly green color, and are therefore objectionable for many applications. Moreover, these materials will not survive processing with various photographic processing solutions unless they are protected by impermeable overcoats.
Blends of polyanilines with other polymers, or polyaniline dispersions in organic solvents, are described in numerous references, including WO 89/02155 (published Mar. 9, 1989). However, such materials are not useful for photographic elements because they provide highly colored coatings, low conductivity or both, and have poor solution processibility. Additional art describes colloidal dispersions of polyanilines, but the polymers are generally not soluble in water or common organic coating solvents.
Dispersions of polyaniline and a protonic acid such as toluenesulfonic acid are described in Japanese Kokai 02/282245 (Fuji) for use as antistatic compositions. Such dispersions can be prepared by mixing a commercially available "Conductive Paint" (available from Americhem, Cuyahoga Falls, Ohio) with one or more binder materials (such as polymethyl methacrylate, polyethyl methacrylate, polycarbonates, cellulose esters, polyvinylformal or blends thereof) in suitable organic solvents. The formulations, at 2-5% solids, can be coated on conventional polymeric supports to provide electrically conductive layers. Moreover, such layers can be overcoated if desired.
It is also known that organic solvent soluble polyanilines can be produced by the use of selected protonic acids to protonate the polyaniline see for example, WO 92/20072 (published Nov. 12, 1992), WO 92/22911 (published Dec. 23, 1992), WO 93/05519 (published Mar. 18, 1993), WO 93/15510 (published Aug. 5, 1993), EP-A-0 545 729 (published Jun. 9, 1993), U.S. Pat. No. 4,983,322 (Eisenbaumer), and U.S. Pat. No. 5,196,144 (Smith et al)!. The solvents or mixtures of solvents described for these materials are not generally useful in coating processes used in the preparation of imaging elements either for environmental or cost reasons.
Hence, there remains a need for solubilized conductive polyanilines that can be applied to imaging and other types of elements directly from water or other acceptable common coating solvents in low coverages to provide effective electrically conductive layers.